Cedar image for docker

Dockerfile + instructions on how to generate cedar docker images. I'm trying to have them at https://index.docker.io/u/heroku. A preview of images generated by these changes can be seen here: https://index.docker.io/u/fabiokung.

Once we have this Dockerfile merged, we will also be able to setup [Trusted Builds](http://docs.docker.io/en/latest/use/workingwithrepository/#trusted-builds).